This invention relates to a breaker system for high viscosity fluids and method of use. More particularly this invention relates to a breaker system for high viscosity fluids and a method of use in treating subterranean formations. Most particularly, this invention relates to a secondary amine breaker system and method of use in fracturing subterranean formations.
Fluids exhibit a measurable property known as viscosity, a term which may be broadly defined as the internal friction or molecular attraction of a given material which manifests itself in resistance to flow. It is measured in liquids by standard test procedures and is usually expressed in poises or centipoises at a specified temperature, although in oil technology it is sometimes expressed in seconds required for a given volume to flow through a specified orifice at a definite temperature. The viscosity of a fluid is an indication of a number of behavior patterns of the liquid at a given temperature including pumping characteristics, rate of flow, wetting properties, and a tendency or capacity to suspend an insoluble particulate material.
There are a number of industrial operations wherein it is desirable to employ viscous aqueous solutions. A very commonly employed viscous aqueous solution is a "water-based fluid" which generally comprises an aqueous fluid of water, brine, seawater, etc., that contains a thickening agent or viscosity builder. When the viscous water-based fluid contains a cross-linked viscosity builder, it is known as a crosslinked water gel. For example, they are employed to fracture subterranean formations penetrated by a borehole for increasing the production of petroleum fluids, namely crude oil and natural gas. Viscous aqueous solutions are also employed in the secondary recovery of oil from oil bearing subterranean formations by fluid drive processes. There are many drilling fluids that are based upon viscous aqueous liquids. Viscous aqueous solutions are also used as carrier fluids in sand control operations such as emplacement of a gravel pack or consolidated gravel pack, and the like, particularly where low fluid loss and/or particulate transport properties are desired. Viscous aqueous fluids are also employed to transport solids through pipelines and the like.
In all of these processes it is often times desirable to control the viscosity of the viscous aqueous solution. Generally, it is desired to reduce or "break" the viscosity of the fluid after a period of time. The reduction of the viscosity of a liquid results in a corresponding reduction in the capacity of that liquid to suspend particulate material, resulting in the dropping out or settling out of the particulate materials suspended in the fluid.
For example, in fracturing a subterranean formation, a fracturing fluid (usually a liquid, although some gas may be present), having granular insolvable particulate material suspended or slurried therein, is injected down a well penetrating the formation and forced back in the formation (employing the height of the hydraulic head of the liquid to provide pressure and hence is commonly known as hydraulic fracturing). Through the instrumentality of such operation, the formation is caused to crack or fracture thereby effectuating improved communication between a fluid (oil, gas, water, etc.) in the formation and the well. Fracturing is customarily conducted by employing oil, water, (sometimes in a thickened or gelled state) or an emulsion thereof which has suspended therein, particles which are substantially insoluble in the liquid and the fluids of the formation, at least a portion of the particles thereby being forced to lodge in the fractures created, thus propping open the fractures when the fracturing pressures are subsequently released and the well put back in production.
To maintain a propping agent suspended in a liquid unit until it is lodged in a fracture presents problems since the propping agent tends to settle out as the rate of movement of the liquid, after leaving the wellbore, slackens. This sometimes occurs prematurely resulting in depositing the propping agent in the wellbore or immediately adjacent to the wellbore, causing what is known as a sand-out or a screen-out. In efforts to avoid the premature deposition of the propping agent, liquids have been used, the suspending ability of which is improved by thickening the same with a suspending agent (a viscosity increasing agent). This approach to the problem, though aiding and maintaining the propping agent suspension for additional time, introduces the added problem of tending to prevent the propping agent from lodging securely in the newly created fractures and to prevent the removal of the fracturing liquid from the fractures. It would be desirable to "break" the viscosity of the fracturing fluid with a "breaker" to reduce the capacity of the fracturing fluid to maintain the propping agents in suspension. Breaker generally refers to chemical additives to viscous water-based fluids which will reduce the viscosity of the fluid after a delayed period of time.
The use of "breakers" to reduce the viscosity of aqueous gelled treating fluids is not new and has heretofore been accomplished using a variety of techniques such as by the degradative action of acids, enzymes, or mild oxidizing agents. However, many of the prior art breakers were limited to temperatures of at least 100.degree. F., and generally were only effective on certain specific aqueous solutions. The lack of utility of the prior art breakers at lower temperatures meant that the breakers were frequently less than satisfactory for application to the special case of relatively low temperature oil and gas reservoirs (defined as from about 50.degree. to about 125.degree. F.). For example, as the gelling agent or viscosity builder concentration is increased, the water gel or fracturing fluid requires extended times to effect a complete break. The delay caused by the inability to affect a rapid and complete break under relatively cool reservoir conditions, disadvantageously extends the well clean-up time, i.e. retraction of water gel or fracturing fluid from the wellbore, and in some cases inhibits the satisfactory clean up operation. Consequently the art has tended to favor modified techniques which are designed using light viscosity fracturing fluids to circumvent or partially circumvent the unsuitable nature of the more viscous, more frequently cross-linked gels or fluids. Such modified techniques produce the undesirable result of a lowering of the permissible proppant concentration, coupled with the need for substantially increased pumping rates. Recent studies have shown that the importance of high total proppant volume in good stimulation treatments and the importance of controlling pumping rates for good fracture high control.
In an effort to overcome the limitations of the prior art breakers, it was asserted that the breaker disclosed in U.S. Pat. No. 4,560,486 and U.S. Pat. No. 4,250,044, would promote the reduction of the viscosity of viscous water based fluids maintained at temperatures within the range from about 50.degree. F. to about 125.degree. F., wherein the fluid comprised a polysaccharide thickening agent, and as a breaker a persulfate plus a tertiary amine. However, the breaker disclosed in the '044 patent and '486 patent was recognized as not being able to break certain cross-linked gels without the addition of a suitable base to adjust the pH to within a critical necessary range, because many times with crosslinked water gels the crosslinking is reversible if the fluid is adjusted to an acid pH.
It is therefore an object of the present invention to provide a breaker that is able to effect a complete break of a cross-linked gel in a relatively low temperature of oil and gas recevoir without the addition of a base to adjust the pH within a critical range.